Expose runtime baking functionality in OccluderInstance3D

[Calinou]

This can be used in editor plugins and exported projects.

• This closes Allow Baking OccluderInstances from script godot-proposals#4316.

Testing project: test_occluder_runtime_bake.zip

TODO

• Move saving to disk to a different method (e.g. bake_scene_and_save()), so you can bake without saving to disk. This can be useful for procedurally generated levels that are discarded after a gameplay session, so caching wouldn't be relevant here.

[Mickeon]

Docs are fine now and the feature is intriguing by itself, but the TODO is intriguing, as well:

Move saving to disk to a different method

[RadiantUwU]

i need this for my game... 😭

[clayjohn]

Looking at the proposal it seems that the user was just asking for the simplify function to be exposed. They are already able to bake occluder nodes at run time, but they are missing the ability to simplify the generated mesh. Accordingly, I wouldn't go forward with exposing the bake function as-is, instead, to solve their problem, we should be exposing the simplify function in an easier to use way.

The problem with the bake function is that it does a bunch of additional things that will be harmful:

1. It recursively walks over the entire sub scene
2. It stalls the GPU by retrieving mesh data from the GPU
3. it saves the resource (which is really bad for procedurally generated scenes)

[Calinou]

They are already able to bake occluder nodes at run time, but they are missing the ability to simplify the generated mesh.

"Bake" isn't 100% correct here, as the user has to manually create the ArrayOccluder3D and set its vertex data by manually traversing the scene. This is still nontrivial to do correctly for users (especially when multimeshes are involved), so I think a high-level bake() function makes more sense to expose. I agree baking and saving should be separate functions for sure, and that simplification can be exposed separately (most likely in Mesh).

Stalling the GPU is inevitable either way (it will also happen if you manually query mesh data in your script), but it's expected that you only bake occluders when loading a scene, not during gameplay. The operation takes too long to be viable during gameplay anyway, and changing occluder data triggers an Embree BVH rebuild which further slows things down.